To interrupt Barry and David for a moment.
I still see UKIP as the protest party for those that will not vote for the other two major parties, they have taken over from the Libdems who had previously held that position.
Having said that I also think that they could grow into a major party mainly because of their anti immigrant stance that is so popular with many where there is a large immigrant population that are perceived as taking 'our' jobs or those who still have not integrated with 'our' ways and want to inflict their dubious customs here.